In various situations thin light fixtures are desirable. For example, back-lit flat panel displays normally should be kept as thin as possible. When they are thinner they tend to be lighter in weight and more compact, both desirable properties when they are used in applications such as portable computers. Automobile taillights and brake lights also should be kept compact where possible. This is because space used by such lights comes at the expense of luggage space in the vehicle's trunk.
Various designs for thin panel lighting have been proposed. For example, copending, commonly assigned U.S. patent application Ser. No. 016,858, filed Feb. 20, 1987, now U.S. Pat. No. 4,789,921 describes a Fresnel-type reflector lying on a conic surface. Typically the Fresnel-type structures of that reflector are designed to mimic the performance of a parabolic reflector. The conic Fresnel provides a highly efficient compact light source, but suffers from the same disadvantage as other parabolic reflectors. That disadvantage is a lack of uniformity in intensity of light output. As may be expected, regions closer to the light source will be more brightly illuminated than those more distant from the light source.
Other designs have been proposed for use when uniform illumination is desirable. One such proposal is described in commonly assigned, copending U.S. patent application Ser. No. 030,033, filed Mar. 24, 1987 now U.S. Pat. No. 4,799,137. That design utilizes a film known as right angle film. The use of right angle film in a light fixture allows the output optical window to be very evenly illuminated, but requires the separate construction of the right angle film and the insertion of that film into the light fixture. In some situations it would be desirable to construct a light fixture having a performance similar to that of the fixture using right angle film, without the requirement of the insertion of a separate film into the fixture.